Light-emitting diodes (LEDs) are increasingly used in modern display technologies. Extension to solid-state lighting (SSL) based on LEDs promises significant advantages over conventional lighting sources such as incandescent and fluorescent lights, while at the same time presenting fewer environmental issues and offering unique lighting design opportunities. LEDs' advantages over traditional light sources include low energy consumption, long lifetime, robustness, small size and fast switching. However, LEDs remain relatively expensive and require precise current and heat management relative to traditional light sources. Fabrication costs are significant and exceed the material costs for some LEDs.
SSL development has focused on LED devices from either inorganic or organic semiconductors, referred to as conventional LEDs or organic light-emitting diodes (OLEDs), respectively. Conventional LEDs are from AlGaAs (red), AlGalnP (orange-yellow-green), and AlGaInN (green-blue), which emit monochromatic light of a frequency corresponding to the band gap of the semiconductor compound used. These conventional LEDs do not emit light of mixed colors, for example, white light. White LEDs can be used as light sources and are capable of producing full color displays with existing color filter technology. One method to produce white light is to combine individual LEDs that simultaneously emit the three primary colors, which mix to produce white light. Another method to produce white light is to use a yellow phosphor to convert monochromatic blue light, or two or more phosphors emitting different colors with blue or UV light. Although LED to broad-spectrum white light is possible in this manner, color control is limited.
Conventional LED technology is considerably more mature than OLEDs; however, the low potential cost of producing OLED panels using high throughput roll-to-toll processes makes OLED technology very attractive for SSL applications. State of the art LED devices that are “warm” white, which is common of incandescent lighting, are considerably lower in efficacy than those that are “cool” white, which is common of florescent lighting. OLEDs are more conducive to achieving “warm” whites. Combination with other potential advantages, such as, ease of thermal management, compatibility with flexible substrates, and fabrication of transparent devices; has allowed OLED technology to receive significant R&D investment. Although OLEDs can also be fabricated relatively inexpensively and provide a variety of colors and “warm” white light, OLEDs generally suffer from deficiencies in efficiency and lifetime relative to LEDs because of the organic material. The relatively high current density and driving voltage required to achieve high luminance promote degradation of the OLEDs, especially in the presence of oxygen, water and UV photons.
Significant increases in OLED device performance and color quality are required to achieve and exceed the DOE SSL 2015 Multi-year Program Goals of 125 lm/W performance with a Color Rendering Index (CRI) quality of above 90. Light extraction is a significant issue for OLEDs, as only 30-40% of the light emission generated in the OLED emissive region can escape the device in the forward viewing direction even when employing state-of-the-art light extraction enhancement methods. Typically, organic molecules provide relatively broad emissions, with spectral full-width-at-half-maximum (FWHM) values typically ranging from 70 to 100 nm. Presently, to achieve a CRI in excess of 90, an appreciable amount of red emissions, at wavelengths approaching 700 nm, is required. Therefore, achieving a broad-spectrum white light source from OLEDs in an efficient manner remains a desirable target.